I've looked across the internet, and haven't found any reports of someone else doing this. So I'll just put this up as a guide. This works for me, but I can't guarantee it'll work for you. I also take no responsibility for any damage done to your Wii
Step 1) download MulticIOS installer http://www.badongo.com/file/16888682 and Softchip http://merge.it/softchip
Step 2) Use MulticIOS installer to install IOS37 with IOS37 dip as IOS232
Step 3) Use Softchip to launch Guitar Hero 5
Step 4) ENJOY!
This also works to get Rock Band 2 running with USB instruments working
Step 1) download MulticIOS installer http://www.badongo.com/file/16888682 and Softchip http://merge.it/softchip
Step 2) Use MulticIOS installer to install IOS37 with IOS37 dip as IOS232
Step 3) Use Softchip to launch Guitar Hero 5
Step 4) ENJOY!
This also works to get Rock Band 2 running with USB instruments working